Australia's rainforests may only cover 0.3 per cent of our total land area, but scientists are discovering they boast a magnificent natural bounty, particularly of fruit.

Stretching from the wet tropics in the north, through pockets in New South Wales and Victoria all the way down to Tasmania, our rainforests are home to an astonishing range of animals and plants — over 10 per cent of Australia's plant species and up to 50 per cent of its animals.

"Between 2000 and 3000 [plant] species have been named and described," says Dr Maurizio Rossetto, conservation geneticist at the Botanic Gardens Trust in Sydney. "But we are still discovering new species, including large trees."

It's the fruit of some of these unique plants that have got scientists most intrigued. Many are jam-packed with health-promoting antioxidants, some in even greater concentrations than traditional sources of these compounds, like blueberries. A few have also shown cancer-fighting properties.

Nature's health foods

The past decade has seen a big increase in the consumption of 'functional foods', says innovative food technologies researcher Dr Roger Stanley of Queensland Primary Industries and Fisheries.

"That is, foods that are eaten not just to fight hunger, but also to boost the immune system, fight ageing and generally promote wellbeing," he says.

The Amazon rainforest was the first place researchers seeking new 'nutraceuticals' started to look, and it gave the world exotic fruits like the acai and cupuacu. It wasn't long before researchers began to show interest in Australia's rainforest fruits.

Although many have been used as Aboriginal bush foods for millennia, we know little about these fruits. This is probably in part because not all of them can be eaten straight from the tree — the chemical compounds that make them so healthy can also make them a bit tart to eat.

Dr Izabela Konczak from Food Science Australia has been investigating 20 native Australian fruits, most of which come from the rainforest.

She is interested in these species for their antioxidant capacity and the presence of phenolic compounds, anthocyanins and ascorbic acid (vitamin C). Antioxidant phenolic compounds are essential for the growth and reproduction of plants, and help defend injured plants against pathogens. Anthocyanins also help protect and repair cells and offer great hope in fighting cancerous cell growth.

Fact file:

When: Australian rainforest species fruit during summer and autumn, however there can be wide variability in the time of year different plants flower and fruit, even within a single species.

Where: Different species of fruit can be found in rainforest pockets from the northern tropics through NSW, Victoria and down to Tasmania. Some other native fruit species found outside these areas, like the Kakadu Plum, have also been found to be rich in antioxidants and vitamin C.

Riberries taste like cloves and spices.
(Source: Galeru)

Konzcak and colleagues found that Davidson's plum, riberries and muntries (also known as emu apples) are all rich sources of antioxidants, comparable to blueberries, while the Kakadu, Burdekin and Illawarra plums possess antioxidant levels three to five times higher than blueberries. The Kakadu Plum (which, as its name suggests, isn't actually a rainforest species) is also the richest known source of vitamin C (ascorbic acid).

"Others that we looked at were very good sources of vitamins and minerals such as folate, iron, zinc, magnesium and calcium, which are very important for DNA replication, cell repair and general health," says Konzcak. "Then, when we tested the Illawarra Plum further, we found that it inhibits the growth of cancer cells, although we are still working on why that is so."

All of the species she collected were from the wild, and this is one of the keys to their strength.

"Most of the fruits we analysed are quite small, with large stones and a thin layer of flesh. The antioxidants and other bioactive compounds have evolved to protect the fruit from pests and diseases, with concentrations eight times higher near the skin than in the centre.

"We know from commercial fruit production that the fruits produced through intensive agriculture, such as apples, have significantly lower levels of these phytochemicals [chemicals produced by the plants] than those that are grown organically," says Konczak.

"This is probably because without fertilizers, pesticides and irrigation, plants have to fight much harder to get their fruit to ripening stage, where it can then be dispersed by animals."

Australia's native bountyRiberries (Syzygium luehmannii): Good source of antioxidants, used in jams as a gourmet bush food.

While some species are able to self-pollinate, others have separate male and female plants that need to grow in close proximity to ensure pollination occurs. Species that pollinate in this way include the Tasmanian pepper, which grows in cool wet forests in Tasmania and as far north as the Blue Mountains in NSW, and the Illawarra plum, which grows in rainforests and near waterways in NSW and Queensland.

Other species, such as Davidson's and Burdekin plums, rely on cassowaries and large mammals like tree kangaroos to eat the fruit and disperse the seeds in their droppings.

"Small red fruits such as riberries, brush cherries and probably the Cedar Bay cherry are spread by small birds, but larger fruits require help from much larger animals," says Rossetto.

"We know that in vestigial pockets of rainforest in northern NSW and Queensland, where there are no big native dispersal animals, these large-fruited species simply aren't there," he says.

Other species rely on chance to survive: the critically endangered Nightcap Oak (Eidothea hardeniana), which is found in only a single creek catchment in the Nightcap Range in northern NSW, depends on the forgetfulness of native bush rats.

The rats collect and hoard the fruit in different locations around their territory. While the rats eat most of the fruit and seeds, the occasional one escapes the rat's attention, and eventually germinates. This hit and miss process has probably been a significant contributor to the tree's vulnerable status.

Saving seeds

Using cryopreservation researchers can store the seeds of threatened rainforest species long-term.
(Source: A. Briggs)

In an effort to conserve our rainforest diversity, Hamilton is collecting and finding ways to store and germinate seeds from these rainforest plant species.

The Rainforest Seed Project is working in partnership with the Millennium Seed Bank at London's Kew Gardens, which aims to bank seeds from 10 per cent of the world's wild plants by next year. Rare and useful species like fruiting rainforest plants are of particular interest to the bank.

Hamilton says seed banking is a cost-effective way of conserving vulnerable species outside of their natural habitat.

"Worldwide, the seed biology of most rainforest species is poorly understood, including seed tolerance to drying and thus capacity to be stored in a conventional seed bank," she says.

Another big challenge to seed collection and conservation is the variability in the time of year that different trees of the same species flower and fruit.

"Sometimes even in a single forest, we will find one stand of plants flowering, one with ripe fruit and one doing absolutely nothing," says the Botanic Garden's Rossetto. "They respond to locally suitable conditions, which vary widely between species."

The reason for this may be because all the trees of one species in one area of the rainforest are often genetically related.

"By staggering fruiting times, closely-related plants are ensuring that there is maximum opportunity for their genetic material to be spread across a wide area."

Despite these difficulties, the seed bank project has so far has tested the seeds from 56 eastern Australian rainforest species.

The team is collating information about the plants in a central database that will be available to other scientists, and seeds will be available in case vulnerable plants need to be replaced in their natural habitat.

The hope is that conservation projects like this will not only help protect these threatened species in the wild, but also help us one day unlock the potential of these fascinating fruits.